The White House has warned that North Korea's threats to train the sights of its rockets at US bases can only isolate the country further.

Leader Kim Jong-Un has signed off on the order to prepare for an attack on American bases in South Korea and the Pacific, according to the North's KCNA news agency.

The move was followed by reports of increased activity at North Korea's mid to long-range missile sites, according to South Korea's Yonhap news agency.

It comes after two American stealth bombers flew over South Korea in a show of force to Pyongyang, following an escalation of rhetoric from the North's young leader.

White House spokesman Josh Earnest has responded by saying the US has the ability and willingness to defend its interests in the region.

Image Caption:Kim at an urgent meeting. To his left, an iMac made by US tech giant Apple

He called on North Korea to end its "bellicose rhetoric", abandon its nuclear programme and live up to international obligations.

Both China and Russia have appealed for calm, with Moscow saying the heightened military activity was slipping into a "vicious cycle" that could spin out of control, implicitly criticising the US bomber flights.

Foreign Minister Sergey Lavrov also suggested the North should cool down, calling on "all sides not to flex their military muscle" and avoid the danger of a belligerent response.

China's foreign ministry spokesman Hong Lei said: "We call on all relevant parties to make joint efforts to turn around the tense situation. Peace and stability on the Korean peninsula as well as Northeast Asia serves the common interest."

The planes were taking part in a joint South Korea-US military exercise that has inflamed tensions with Pyongyang, which earlier this month threatened to unleash an "all-out war" backed by nuclear weapons.

The US has denied its military exercise was provocative but said it was "committed to a pathway to peace" and "prepared to deal with any eventuality" in the region.

KCNA reported that Mr Kim had "judged the time has come to settle accounts with the US imperialists in view of the prevailing situation".

The agency said: "He finally signed the plan on technical preparations of strategic rockets of the KPA, ordering them to be on standby for fire so that they may strike any time the US mainland, its military bases in the operational theatres in the Pacific, including Hawaii and Guam, and those in South Korea."

Following their leader's call to arms, thousands of North Koreans turned out for a mass rally in the main square in the capital.

Chanting "Death to the US imperialists" and "Sweep away the US aggressors," soldiers and students marched through Kim Il-Sung Square during the 90-minute rally.

Earlier, US Defence Secretary Chuck Hagel said: "The North Koreans have to understand that what they're doing is very dangerous.

"I don't think we're doing anything extraordinary or provocative or out of the ... orbit of what nations do to protect their own interests."

The US, he added, must make it clear to South Korea, Japan and other allies in the region that "these provocations by the North are taken by us very seriously, and we'll respond to that".